The present invention relates to cellular telephony and, more particularly, to a searcher for a DSSS cellular telephony system.
In a DSSS cellular telephony system, the base stations identify themselves by transmitting pilot signals. Each pilot signal is a sequence of zero bits, modulated, according to the principles of DSSS encoding, by a pseudonoise (PN) sequence, or an extended pseudonoise sequence.
For example, under the IS-95 interim standard, the PN sequence is 2.sup.15 chips long, with the n-th chip including an in-phase component i(n) and a quadrature component q(n). The initial values of i and q are i(1)=q(1)=1 and i(n)=q(n)=0 for 2.ltoreq.n.ltoreq.15. Subsequent values of i and q, up to n=2.sup.15 -1, are obtained recursively as follows: EQU i(n)=i(n-15)+i(n-10)+i(n-8)+i(n-7)+i(n-6)+i(n-2) (1) EQU q(n)=q(n-15)+q(n-12)+q(n-11)+q(n-10)+q(n-9)+q(n-5)+q(n-4)+q(n-3) (2)
where the additions are modulo 2. Finally, i(2.sup.15)=q(2.sup.15)=0.
The same PN sequence is used by each of the base stations. The base stations are synchronized; and each base station uses the PN sequence with a different delay (also called "PN offset") to produce the pilot signal. This enables the mobile units of the cellular telephony network to distinguish one base station from another.
The total signal received by a mobile station, as a function of time t, is: ##EQU1##
Here, b indexes the B base stations; m indexes the M.sub.b transmission paths (multipath channels) from base station b to the mobile station; C is the channel gain of multipath channel m; .tau. is the additional delay introduced to the PN sequence by multipath channel m; the "1" inside the brackets represents the sequence of zeros that is modulated by the base stations to produce the pilot signals; i indexes the I.sub.b other users that are transmitting via base station b at time t; .alpha. is the power of user i relative to the pilot signal; D is the data transmitted by user i; W is a code sequence (for example, a Hadamard code sequence) that is used in addition to the PN sequence to modulate data D and allow simultaneous transmission on the same physical channel by all the users in addition to the pilot signals; and N is additive noise.
Each mobile unit of the cellular telephony network determines which base station to communicate with (typically, the nearest base station) by correlating this signal with the PN sequence at a set of trial delays. Because data D are modulated by sequences W, the correlation of the part of the signal that comes from other users is negligible. The correlation with the pilot signals also is negligible, except at trial delays that are equal to the PN offsets used by the base stations, as modified by multipath delays .tau.. Specifically, a pilot signal that arrives at a delay, that is equal to the sum of a base station offset and one of the multipath delays .tau. associated with transmissions from that base station, gives a significant contribution to the correlation at a matching trial delay; and all other pilot signals contribute negligibly to the correlation at that trial delay. This correlating is performed when the mobile station powers up, and continuously thereafter, to allow hand over from one base station to another when the mobile station crosses a cell boundary. The delays of the various base stations are well separated, by more than the largest anticipated multipath delay, so in the absence of additive noise and in the absence of multipath delays, only a small number of correlations, equal to the number of potential nearest base stations, would have to be performed, to identify the base station whose delay gives the highest correlation as the nearest base station. According to the IS-95 standard, this separation is at least 256 chip durations T.sub.c. Because the pilot signals and data D are received by the mobile station from each base station via several paths at different delays (PN offset+.tau.), the various replicas of the signals thus received are combined to suppress the deterministic noise represented by the various multipath delays .tau.. For example, maximal ratio combining is the optimal combination method in a bit error rate and frame error rate sense. In order to do this combining, the multipath delays must be determined. Therefore, the correlation is performed at a series of delays in a window centered on the nominal delay. The size of this window depends on the local topography, and is provided to the mobile unit by the base station. One typical window size, according to the IS-95 standard, is 60 chip durations.
FIG. 3 is a schematic block diagram of a mobile station receiver 30. RF signals are received by an antenna 60, down converted to an intermediate frequency (IF) by a down converter 62, filtered by a bandpass filter 64 (typically a surface acoustic wave filter) to eliminate signals outside the required bandwidth, and amplified by an automatic gain control 66. The amplified IF signals are multiplied by an IF sinusoid 65, without (block 68i) and with (block 68q) a 90.degree. phase shift 67, to produce an in-phase signal I and a quadrature signal Q. In-phase signal I is filtered by a low-pass filter 70i and digitized by an A/D converter 72i. Similarly, quadrature signal Q is filtered by a low-pass filter 70q and digitized by an A/D converter 72q. A searcher 80 receives the digitized signals and performs the correlations needed to determine the various multipath delays .tau. inside the target window. The digitized signals are again correlated, at the delays determined by searcher 80, by the correlators of a correlator bank 74, and the outputs of correlator bank 74 are combined, in a maximal ratio sense, in a rake combiner 76 to produce the final output signal.
In order to ensure uninterrupted communication as a mobile station crosses from one cell to another, the correlations performed by searcher 80 must be performed rapidly. In fact, it is not necessary to perform the full correlation at each delay in the window. It suffices to perform a correlation that is only long enough to ensure a high detection probability at the right delay and a low false alarm probability at the wrong delay. Typically, the length of the correlation, measured as a multiple N of the chip duration T.sub.c, is between 500T.sub.c and 2000T.sub.c.
To make the correlations even more efficient, the dual dwell algorithm is used. At each delay in the window, the correlation is performed for a number M of chip durations that is less than N. Only if the correlation value after M chip durations exceeds a certain threshold is the correlation performed for the full N chip durations. The threshold, and the parameters N and M, are chosen to maximize the detection probability while minimizing both the false alarm probability and the time spent correlating. See, for example, M. K. Simon, J. K. Omura, R. A. Scholtz and B. K.
Levitt, Spread Spectrum Communication, Vol. III, Computer Science Press, 1989, chapter 1, particularly section 1.3, and D. M. Dicarlo and C. L. Weber, "Multiple dwell serial search: performance and application to direct sequence code acquisition", IEEE Transactions on Communications vol. COM-31 no. 5 pp. 650-659, May 1983. In the prior art implementation of this algorithm, several correlators are used by searcher 80 to correlate the received pilot signal with the PN sequence at several adjacent delays in the window. If none of the correlation values exceeds the threshold after M chip durations, then the correlators are used to correlate the received pilot signal with the PN sequence at the next several adjacent delays. If at least one of the correlation values exceeds the threshold after M chip durations, then all the correlations are continued for the full N chip durations, but only the correlation values obtained by the correlators whose correlation values exceeded the threshold after the initial M chip durations are actually considered. The brute force approach to reducing search time, adding more correlators, is inefficient, because the more correlators that are used, the more likely it is that one of the correlators passes the threshold. In that case, the other correlators, which did not pass the threshold, continue to correlate unnecessarily for the full N chip durations.
There is thus a widely recognized need for, and it would be highly advantageous to have, a configuration for a cellular telephony searcher that would allow the efficient use of many correlators.